Artificial ground freezing is a technique that has been used for a variety of purposes. Most commonly, ground freezing is used to provide support or containment for excavations or to provide subterranean barriers against water seepage or the spreading of contaminants. Frozen underground barriers have also been formed to create impermeable zones for use in the extraction or processing of minerals.
A typical ground freezing system involves drilling a bore in the earth and lining it with a steel freeze pipe. Refrigerant is pumped to the base area of the freeze pipe through a smaller feed pipe that extends within the freeze pipe. As the refrigerant flows upwardly within the freeze pipe, it extracts heat from the earth around the bore and eventually freezes the interstitial water to create an area of frozen soil that can exhibit compressive strength as great as some types of concrete. a number of bores are strategically situated such that the frozen zones from adjacent bores merge to form a continuous barrier.
In some ground freezing applications, it is necessary to create frozen barriers that extend to considerable depths. For example, ground freezing to depths of 1000 feet or more has been achieved. However, the need to freeze the ground to substantial depths leads to complications. One example is that thermal contraction effects become increasingly pronounced as the pipes increase in length. The feed pipes are typically constructed of high density polyethylene (HDPE). HDPE has a relatively large coefficient of thermal expansion, so long pipes contract significantly when cooled by the refrigerant. As a result, the bottom discharge end of the pipe can rise so much that the lower end portion of the freeze pipe fails to receive refrigerant. Then, the lower part of the bore remains unfrozen.
Although metals such as steel contract much less than HDPE due to thermal effects, such metals are also much heavier and must be supported from the bottom. However, when long metal feed pipes are cooled, they contract enough to rise above conventional bottom support systems such that the pipe weight is then borne by a fitting at the top end of the feed pipe. This can exert undue stress on the top fitting and cause the feed pipe to detach from it, resulting in a structural failure.